1. Field of the Invention
The present invention generally relates to the recovery of messages from an angle modulated carrier consisting of either frequency modulation (FM) of a carrier signal or phase modulation (PM) of a carrier signal.
A modulated sinusoidal carrier is used to transfer messages in radio and telecommunication systems. The message in modern systems causes either the frequency, using FM, or the phase, using PM, of the carrier to vary in accordance with the amplitude (voltage) of the message signal. In the case of analog messages, such as voice in Advanced Mobile Phone Systems (AMPS) cellular systems, for example, the preferred choice of modulation at present is FM. Digital messages, used in digital cellular systems and for computer communications, are commonly carried by some form of PM or by FM on occasion.
Recovery of the message from a modulated carrier in the case of FM is accomplished with an analog circuit such as a phase locked loop or a quadrature demodulator. Demodulation of PM carriers involves the recovery of bit streams since analog messages are not transferred using PM. Such PM demodulation requires a coherent reference signal. Typically, the PM carrier is separated into in-phase and quadrature components and these components are demodulated separately by multiplying each of these components by a coherent reference signal with subsequent lowpass filtering of each multiplier output. The coherent reference must either be derived from the received modulated carrier, or be obtained by trial and error or by a search routine.
A present trend is to replace many of the analog operations of a receiver, including demodulation, with digital circuitry. Use of digital circuitry provides design flexibility because changes in circuitry operation can be made with code changes without changing hardware. Realizing operation of phase locked loops as demodulators or realizing operation of quadrature demodulators with digital circuitry is difficult because of the large number of digital operations and the large amount of memory required. Similarly, the large number of multiplication and filtering functions needed in coherent demodulation of PM carriers requires involved digital circuitry and code, both of which compromise the speed of processing. Therefore, there is a need to develop methods and apparatus that permit demodulation of either FM or PM carriers with simple digital circuitry and without the need for a coherent reference.
2. Prior Art
U.S. Pat. Nos. 4,859,958 and 4,992,747, invented by Glen A. Myers, the inventor of the present invention, are each incorporated by reference in the present application as though fully set forth herein. In these patents, a means for demodulating all of several co-channel FM carriers is described.
U.S. Pat. No. 5,038,115, co-invented by the inventor of the present invention, is also incorporated by reference in the present application as though fully set forth herein. In this patent, phase tracking of input terminal signals is described. In one embodiment of the phase tracking circuit of U.S. Pat. No. 5,038,115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed forward manner.
U.S. Pat. No. 5,329,242 invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In this patent, demodulating a frequency modulated signal using the time intervals between zero crossings of a received carrier signal is described. Averaging and mapping techniques are used to improve estimates of the message signal.
U.S. Pat. Nos. 5,541,959 and 5,570,395, invented by the inventor of the present invention, are also incorporated by reference as though fully set forth herein. These patents describe, analytically and geometrically, the effect of adding two sinusoids of different frequency.
U.S. Pat. No. 5,606,581, invented by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application described a method and apparatus for creating a replica of a dominant carrier.
U.S. Pat. No. 5,554,955, invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. This patent describes a method and apparatus for removing the effects of co-channel interference from the message on a dominant frequency modulated carrier and for recovering the message from each of two co-channel carriers.
U.S. patent application Ser. No. 08/705,721 now U.S. Pat. No. 5,878,084 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes a method and apparatus for recovering the independent bit streams from each of two co-channel frequency modulated carriers.
U.S. patent application Ser. No. 08/870,469 now U.S. Pat. No. 5,878,084 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes a method and apparatus for transferring two or more independent bit streams using each of two co-channel frequency modulated carriers.
U.S. patent application Ser. No. 09/045,225 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes a method and apparatus for using feature characteristics including times of occurrence to represent independent bit streams or groups of bits in data transmission systems.